Set the default featured image

Set the default featured asset from an image asset added to the story.
This mirrors the logic that occurs when retrieving the featured image on
the front end.

This happens via event listeners in ``FeaturedAssetView`` that checks when
the first image asset is added to the story and sets it as the featured
image.

To work around a race condition, I started having ``FileUploadMixin``
trigger a ``fileupload`` event on the model once it has successfully had
its related file uploaded.

Addresses #515.
Workaround for featured asset race condition

The previous workaround was not working on non-local machines.
The ``fileupload`` signal is also no longer required.
Explicitly pass changed event to callback
Set Tastypie dependency version

Set the Tastypie dependency verison to before backward-incompatible changes
were added.
Merge branch 'develop' of github.com:PitonFoundation/atlas into develop
Fix typo

The commit SHA was wrong for the required commit of django-tastypie
Test for automatic featured image selection
Listen to "change:status" not "publish:story"

Defining a custom event on the event bus isn't neccessary because the
"change:status" event is triggered on the story model when it's
published/unpublished.
Fix display of sharing sidebar

The toggling with story publish/unpublish got broken in previous commits.
Remove debugging statement typo.